Process and device for introducing bulk material into a rotary hearth furnace

ABSTRACT

A process for feeding bulk material onto a belt of a rotary hearth furnace, which has hoods that cover the hearth belt in such a manner as to form a ring. 
     The bulk material is deposited on a transport device at a layer thickness proportionately dependent on the distance to the rotary hearth center. The transport speed of the transport device is set at a certain value; the surface of the bulk material is smoothed; and then the bulk material is distributed at a constant layer height over the entire width of the hearth belt.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for feeding bulk material onto a beltof a rotary hearth furnace that has hoods which cover the belt 58 as toform a ring, as well as a suitable device for this purpose.

2. Discussion of the Prior Art

Furnaces of this type demand an even temperature in the furnace chamberat the lowest possible consumption of energy. The removal device and,especially, the charging device, are of particular importance in thisregard. From DE 33 12 467 C2, for example, a furnace for heatingmaterial is known that has a transport device in the area of the dooropening for the purpose of feeding and removing the material, as well asa plurality of gripping devices that can feed the material to be heatedfrom the transport device through the furnace openings to the contentsof the hearth support located in the furnace, or remove the material.This known furnace is a so-called hearth bogie furnace, i.e., one inwhich material is not transported on a circular conveyor.

From EP 0 058 736 B1, a rotary hearth furnace with a heating device toproduce a hot zone is known, which has a device that constitutes afeed-in station, via which parts can be introduced into the furnace.

Neither of these devices makes it possible to place bulk materials ontothe furnace belt. Such a device is known from EP 0 259 510 B1. In thiscase, a screw device is used; however, this device is used to carry awaythe material.

Here, too, as is generally the case, the feeding of the bulk material iscarried out by means of transport belts or chutes. In the present case,the material is passed onto the hearth belt via a lid (not describedfurther).

SUMMARY OF THE INVENTION

The object of present the invention is to provide a process and acorresponding device for covering the belt of a rotary hearth furnacewith bulk material at a very low layer thickness relative to the beltwidth, in a careful and reliable manner.

Pursuant to this object, and others which will become apparenthereafter, one aspect of the present invention resides in a process forfeeding bulk material onto a belt of a rotary hearth furnace which hashoods that cover the hearth belt so as to form a ring. The processincludes the steps of depositing the bulk material on a transport deviceat a layer thickness proportionally dependent on a distance to a centerof the rotary part so that a cross-sectional area of the bulk layermaterial conically tapers to the center of the hearth, setting thetransport device to a speed at least three times greater than that ofthe hearth belt, smoothing the surface of the bulk material on thetransport device, and distributing the bulk material on the hearth beltat a constant layer height over the entire width of the hearth belt.

Another aspect of the invention resides in a device for feeding bulkmaterial onto a hearth belt of a rotary hearth furnace. The deviceincludes hoods that cover the hearth belt so as to form a ring. The ringhas a cutout at a feeding point of the rotary hearth furnace. Means areprovided for transporting the bulk material. The transporting means arearranged above the hearth belt and have a material discharge edgearranged perpendicular to a direction of movement of the hearth belt.The transport means operate at a transport speed which can be set,relative to the hearth belt speed, inversely proportional to layerheights of the bulk material. Furthermore, means are arranged in an areaof the discharge edge of the transport device for smoothing a surface ofthe bulk material. The smoothing means is arranged at an angle relativeto a middle of the rotary hearth furnace above the transport device soas to define a constant cross-sectional area of the bulk material.Finally, means are arranged in front of the smoothing means in thetransport direction for depositing the bulk material in a cross-sectionthat tapers conically to the hearth center.

According to the invention, the feed-in device is constructed so thatmaterial supply is divided up into different steps. The individual stepscan be easily monitored and the individual step transitions reliablycontrolled.

In a first step, material is supplied evenly in a constant amount. Forexample, a movable transport belt is used, the discharge quantity ofwhich is monitored and the discharge position of which is adjustable.The position of the turnover point is controlled in such a way that amaterial layer, the height of which depends on the structure of therotary hearth furnace and, here, on the size of the furnace belt, isestablished (Step 2) on a further transport device. The fact that theindividual belt positions have different transport speeds depending ontheir distance to the rotary hearth center is taken into account. Thelayer height on this transport device is thus highest at the outside ofthe furnace belt, which is most distant from the furnace center. Alltold, the layer height has a cross-sectional area in the form of aquadrangle that narrows conically toward the furnace center.

The exact surface form is established by a smoothing device. Thistransport device is operated at a predeterminable speed. By means of thesmoothing device and the constant speed, it is possible to set a fixedtransport quantity, which is established directly from its position inthe cross-sectional area of the discharge relative to the distance tothe furnace center.

In the third step, the material is carefully discharged from theaforementioned transport means onto the hearth belt at the dischargepoint. The speed of the hearth belt relative to the speed of thetransport means now determines the layer height; specifically, in aconstant fashion over the full width of the furnace belt.

By simply maintaining the conical cross-sectional area of the bulkmaterial on the transport belt, it is possible, because of the exclusivedependence of the speeds of the furnace belt and the transport belt, toconstantly set the layer height in an especially low and reliablefashion.

In practice, layer heights of only 30 mm are demanded on the transportbelt for furnace belt widths of up to 7 m. Such high accuracy isattained by the device according to the invention, with layer heightsfrom 200 to 400 mm being provided on the transport belt. The deviceaccording to the invention is particularly suitable for large-grain bulkmaterials.

Instead of a movable continuous belt, a swivelling belt can also be usedas the first material supplier.

Advantageously, a continuous belt is used as the transport unit. Thiscan have edge-shaped elevations directly on its edges; or else a sideslat is provided, regardless of the transport belt. Given this sidelimit, it is possible to place bulk material on the transport belt atexactly the height desired.

Depending on the material quality, it is also possible to carefullytransport the bulk material by means of a vibrating conveyor that has afixed strip on the side. The vibrational frequency is set so that theconical cross-sectional area is maintained.

An example of material distribution on the furnace belt is illustratedby the following example:

For example, if a furnace belt with a width of 6 to 7 m is used for arotary hearth furnace having a diameter of approximately 35 m, in 6total segments of a cut-out, for instance, a circular cut-out ofapproximately 3°, then the sixth and outermost segment is covered by abulk weight of roughly 36 kg and a total weight of only around 25 kg,corresponding to roughly 70%, is required in the inner segment.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention is shown in the accompanying drawings. Thedrawings show:

FIG. 1 is an overview of a rotary hearth furnace.

FIG. 2 The diagram of the feeding device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a rotary hearth furnace 11 with a midpoint M, the materialtransport belt 13 of which is covered by a hood 14.

A hood cut-out 12 is provided for the feeding device.

FIG. 2 shows a section of the hearth belt 13, which moves at the speedV_(D). The hearth belt 13 is covered by bulk material at a height h_(D).

Above the hearth belt 13, there is a continuous belt 21 of a transportdevice 20 covered by with the bulk material which has a height h_(iF) onthe side toward the center of the rotary hearth furnace; a height h_(mF)in the center; and a height h_(aF) in the outer area. The continuousbelt 21 moves at the speed v_(F) which is at least three times greaterthan the hearth belt speed.

The surface of the material is passed under a smoothing roller 31 of asmoothing device 30 arranged at an angle. Below the underside of thesmoothing roller 31, there are limits in the form of limit slats 24 inthe edge area of the continuous belt 21. On the smoothing roller 31,distributed over its axial length, there are elastic blades 32, whichserve to distribute the material on the continuous belt 21. The belt 21feeds the material to a discharge edge 22 where the material falls tothe hearth belt 13.

In the transport direction of the continuous belt 21, in front of thesmoothing roller 31 and above the continuous belt 21, there is atransport belt 41, and a drive 42 of a material supply device 40. As thearrows 50 indicate, the transport belt 41 can be moved back and forth.

In addition, the transport belt 41 can be swivelled around an axis lyingoutside of the rotary hearth furnace. This is also indicated in thedrawing by a double arrow 60. This swivelling serves to improve thematerial supply onto the continuous belt; particularly, to achievedifferent bulk heights.

What is claimed is:
 1. A process for feeding bulk material onto a beltof a rotary hearth furnace which has hoods that cover the hearth belt soas to form a ring, comprising the steps of:depositing the bulk materialon a transport device at a layer thickness proportionately dependent ona distance to a center of the rotary hearth so that a cross-sectionalarea of the bulk material layer conically tapers to the center of thehearth; setting the transport device to a speed at least three timesgreater than that of the hearth belt; smoothing the surface of the bulkmaterial on the transport device; and distributing the bulk material onthe hearth belt at a constant layer height over the entire width of thehearth belt.
 2. A process as defined in claim 1, wherein the depositingstep includes depositing the bulk material on the transport device inlines in a form dependent on the layer thickness.
 3. A process asdefined in claim 1, wherein the smoothing step includes pushing the bulkmaterial into troughs.
 4. A process as defined in claim 1, wherein thesmoothing step includes vibrating the bulk material.
 5. A device forfeeding bulk material onto a hearth belt of a rotary hearth furnace,comprising:hoods that cover the hearth belt so as to form a ring, thering formed of hoods having a cutout at a feeding point of the rotaryhearth furnace; means for transporting the bulk material arranged abovethe hearth belt and having a material discharge edge arrangedperpendicular to a direction of movement of the hearth belt, thetransport means having a transport speed which, relative to the hearthbelt speed can be set inversely proportional to layer heights of thebulk material; means arranged in an area of the material discharge edgeof the transport device for smoothing a surface of the bulk material,the smoothing means being at an angle relative to a middle of the rotaryhearth furnace above the transport device so as to define a constantcross-sectional area of the bulk material; and means arranged in frontof the smoothing means in the transport direction, for depositing thebulk material in a cross-section that tapers conically to the hearthcenter.
 6. A device as defined in claim 5, wherein the transport meansincludes a continuous belt having edges, a limit being provided at theedges at a height corresponding to the bulk material height.
 7. A deviceas defined in claim 5, wherein the transport means includes a vibratingconveyor and limit slats arranged in an edge area of the conveyor at aheight corresponding to the bulk material height.
 8. A device as definedin claim 5, wherein the smoothing means includes a roller arrangedparallel to the material discharge edge and driveable counter to thetransport direction of the bulk material.
 9. A device as defined inclaim 8, wherein elastic blades are mounted to the outer surface of thesmoothing roller.
 10. A device as defined in claim 5, wherein thesmoothing means includes a strip arranged parallel to the materialdischarge edge.
 11. A device as defined in claim 5, wherein the bulkmaterial depositing means includes a swivelling transport belt having aswivelling drive, the transport belt having an adjustable swivellingangle and swivelling speed.